So now let's look at an example of sex linkage.
Recall that when we say sex linkage, we are talking about an allele carried on the sex
chromosomes.
Females have two Xs and a male has an X and a Y.
But the Y chromosome is much smaller than the X and therefore has fewer alleles.
So one of the examples of sex linkage we talked about earlier was red-green color blindness.
And so for red-green colorblindness, a female would carry two alleles because
she's got a spot for them on both of her X chromosomes.
But a male would only have one, so once we know a male's phenotype we would know the
genotype for sex linkage.
So we were using a capital C to stand for normal color vision and the lower case C to
stand for a red-green color blindness and usually the lower case C is underlined when
we draw just so that we can tell the difference between the upper case and lower case.
So a female who has normal vision could be homozygous, or she could also be heterozygous,
which means she's a carrier of the trait.
A color blind woman would have to have a color-blind allele on each of her X chromosomes and a
male who has normal vision would only have one allele on his X chromosome and that would
be a capital C. If he has color blindness he would have
a lower case c, or a colorblind allele on his X chromosome.
Recall that males get there X chromosomes from their mothers and their Y chromosomes
from their fathers.
So let's combine sex linkage with another trait that we know is simple Mendelian inheritance.
In humans, dimples are dominant over non dimples and red green colorblindness is recessive
and sex linked.
A father is heterozygous dimpled and has red green color blindness and a mother is heterozygous
for both traits.
So let's look at their genotypes.
The father is heterozygous dimpled which means he has a dominant allele and a recessive allele.
And he has a red-green color blindness.
Now since he's a male we have X and Y chromosomes and what we will put on that X chromosome
will be a lower case c because he has redgreen colorblind.
The mother is heterozygous for both traits which means she has a Big D and a small d.
She has two X chromosomes and on one of them she has the dominant allele and on the second
one she has the recessive allele.
So what type of gametes will be these parents be able to make?
So if we look at the dad for example we have to look at every possible combination of the
D and a sex chromosome.
So he would have a possibility of having the dominant allele for dimples and he would have
an X chromosome in some of his gametes and that X chromosome would have a lowercase c.
He could also have the dimpled allele with the Y chromosome which would not be tearing
an allele for red-green colorblindness or for normal color vision.
But he can also have a lower case d because he is heterozygous for dimples.
And he could have with that small d, he could have the recessive allele on his X chromosome
which is the red-green colorblindness allele.
Or he could also have that non-dimpled allele with the Y chromosome.
The mom is also going to have every possible combination of d with x.
So she would have a D, a big D, with a normal vision allele on her X chromosome.
She could have a big D with the same lower case allele for red-green color blindness.
Or she could have a recessive allele for dimples with the normal vision allele on her X chromosome
and she could have the non dimple allele with the red-green colorblind allele.
So this would mean we would need a sixteen box Punnett square.
Notice that we've segregated the alleles the same way we would do even if this was not
a sex-linked trait.
But we use the chromosomes so that we will be reminded that it's an X or Y chromosome.
So let's put those different gametes that they could make along these boxes.
These are from the previous screen.
And then for the male we would put them on the side.
It doesn't really matter which side you put them on.
So let's take a look at the results.
It's a little bit easier to read them already pre-printed.
If we look at the top two rows, we'll see that these are all daughters.
That is because in each one of these boxes we have two X chromosomes.
So these are the daughters.
And the bottom two rows are going to be the sons.
And that is because in each one of these cases we see a Y chromosome.
So when the question asks you "When they have a daughter or when they have a son, what is
the chance that...", whatever the question is, you will look at the appropriate rows
has so that you will not be counting all 16.
But if we asked the question, "What is the chance when they have a child," we would look
at all 16 boxes.
So if we asked you "What is the chance that they could have
a colorblind daughter?"
We would look at the top two rows only and you'll see that there are four out of eight,
and these are shown for you in an orange color.
In the case of a colorblind, non-dimpled son, there's only one out of eight chance that
they would have a son with those two traits.
So the important thing to remember is that in a sex-linked problem if we ask for sons
or daughters we only look at those boxes.
And if we ask for a child, a child having a particular trait, we look at all 16 boxes.